1. Field of Invention
A flow directing device assists the feed of a medium to high consistency slurry through a pump by conditioning the slurry prior to pumping the slurry.
2. Description of Related Art
Pulp and paper mills universally have a major problem moving medium consistency stock (8% to 12% cellulose fibers mixed in water) and high density stock (12% to 18% cellulose fibers mixed in water) from one area of the mill to another. There are several advantages to maintaining the highest possible transport consistency. First, the end product of such mills is paper, not water. Second, mills maintain a stored inventory of stock (e.g. pulp) to run a paper machine when the pulp mill is down and stored water takes up valuable space. Third, the total tonnage of pulp that can be transferred is increased at higher pulp concentrations.
Several basic types of pumps have been used to deliver pulp through piping systems: 1) positive displacement pumps such as rotary lobe and intermeshing screw types, 2) high speed centrifugal pumps, and 3) rotary disc pumps. All of these have advantages and disadvantages.
Positive displacement pumps literally suck the pulp into the suction end and spit it out the other end. These pumps have very close tolerances that must be maintained in order for the pump to operate properly. Pulp must be clean as a small tramp solid, such as a metal staple, can jam the rotating parts, wrecking the pump. Maintenance is very high and expensive. This type of pump has the lowest power cost of operation and the highest maintenance cost of operation.
Centrifugal pumps have poor suction characteristics and depend on an inducer device, attached to the front of the impeller, to drag the medium into the impeller. This inducer creates a vortex in the suction to the pump, leading to the formation of large air bubbles that often block all flow of medium into the pump. These air bubbles must be continuously sucked out of the pump eye by a vacuum system. These pumps are popular because they greatly reduce the high maintenance costs of the positive displacement pumps. However, the unpredictable nature of vacuum systems makes reliable operation of these pumps virtually impossible. Centrifugal pumps are limited to about 121/2% pulp consistency without frequent problems with bridging and clogging.
Rotary disk pumps are relatively new and have many advantages over positive displacement and centrifugal pumps. Rotary disk pumps can pass large tramp solids (up to 3" in diameter), produce no fiber damage, can pump up to 16% consistency, and have very low maintenance requirements. With adequate flow of material, such pumps are capable of handling 18+% consistency stock. However, these still have problems of their own. The major disadvantage is the lack of suction capability. That is, rotary disk pumps can only pump the material that flows into, or is pushed into, the pump. Since pulp has very little gravity flow at consistencies above 10%, this type of pump has serious problems in pumping any higher consistency. Another problem with rotary disk pumps is the "bridging" of stock above the pump suction, effectively blocking all flow into the pump. Vacuum systems have been tested on the backside of rotary disk pumps in an effort to "suck" pulp into the pump. However, this has proven to be unsuccessful.
Several devices have been produced to assist such pumps. Most are various types of inducers (Archimedes screws, beaters, augers, impellers, etc.) attached to the pump impeller shaft. These are commonly in use today to "fluidize" the stock and pull the pulp into the pump suction. However, existing devices are associated with many problems, such as mixing air into the pulp fibers, shearing the pulp fibers, and creating a pump suction vortex.
There is a need for an improved flow directing device that: 1) eliminates bridging of stock, 2) creates a non-vortexing flow, 3) is totally independent from pump operation, 4) focuses flow into the pump suction, 5) minimizes "slip/stick", 6) does not require dilution water, 7) does not mix air into the pulp fibers, 8) significantly improves the flow into the pump suction, and 9) handles entrained air up to 30%, by volume, without problems.